Implantable pump with infinitely variable resistor

ABSTRACT

A variable hydraulic resistor for use with implantable pumps is disclosed. The variable hydraulic resistor according to the present invention is particularly useful in varying the flow rate of a medication fluid from an otherwise constant flow implantable pump. An implantable pump is also disclosed, which does not require a complicated clinching system or the like, and which may include an undulating membrane and chamber design to reduce the height of the pump.

BACKGROUND OF THE INVENTION

The present invention relates to implantable devices, and moreparticularly to a reduced size implantable pump and a programmableimplantable pump allowing for variable flow rates in deliveringmedication or other fluid to a selected site in the human body.

Implantable pumps have been well known and widely utilized for manyyears. Typically, pumps of this type are implanted into patients whorequire the delivery of active substances or medication fluids tospecific areas of their body. For example, patients that areexperiencing severe pain may require painkillers daily or multiple timesper day. Absent the use of an implantable pump or the like, a patient ofthis type would be subjected to one or more painful injections of suchmedication fluids. In the case of pain associated with more remote areasof the body, such as the spine, these injections may be extremelydifficult to administer and particularly painful for the patient.Furthermore, attempting to treat conditions such as this through oral orintravascular administration of medication often requires higher dosesof medication and may cause severe side effects. Therefore, it is widelyrecognized that utilizing an implantable pump may be beneficial to botha patient and the treating physician.

Many implantable pump designs have been proposed. For example, commonlyinvented U.S. Pat. No. 4,969,873 (“the '873 patent”), the disclosure ofwhich is hereby incorporated by reference herein, teaches one suchdesign. The '873 is an example of a constant flow pump, which typicallyinclude a housing having two chambers, a first chamber for holding thespecific medication fluid to be administered and a second chamber forholding a propellant. A flexible membrane may separate the two chamberssuch that expansion of the propellant in the second chamber pushes themedication fluid out of the first chamber. This type of pump alsotypically includes an outlet opening connected to a catheter fordirecting the medication fluid to the desired area of the body, areplenishment opening for allowing for refilling of medication fluidinto the first chamber and a bolus opening for allowing the directintroduction of a substance through the catheter without introductioninto the first chamber. Both the replenishment opening and the bolusopening are typically covered by a septum that allows a needle orsimilar device to be passed through it, but properly seals the openingsupon removal of the needle. As pumps of this type provide a constantflow of medication fluid to the specific area of the body, they must berefilled periodically with a proper concentration of medication fluidsuited for extended release.

Although clearly beneficial to patients and doctors that utilize them,one area in which such constant flow implantable pumps can be improved,is in their overall size. Typically, such pumps require rather bulkyouter housings, or casings, for accommodating the aforementionedmedication and propellant chambers, and septa associated therewith.Often times, implantable pumps are limited to rather small areas withinthe body. Depending upon the size of the patient for which the pump isimplanted, this limited area may be even further limited. For example, aperson having smaller body features, or those containing abnormalanatomy, may present a doctor implanting a constant flow pump with someadded difficulty. Further, patients may be uncomfortable having standardsized constant flow pumps implanted in them. Such pumps are often timescapable of being felt from the exterior of the patient.

Implantable pumps may also be of the programmable type. Pumps of thistype provide variable flow rates, typically through the use of asolenoid pump or a peristaltic pump. In the solenoid pump, the flow rateof medication fluid can be controlled by changing the stroke rate of thepump. In the peristaltic pump, the flow rate can be controlled bychanging the roller velocity of the pump. However, both of these typesof programmable pumps require intricate designs and complicatedcontrolling mechanisms. As such, it is more desirable to utilize pumpshaving designs similar to the aforementioned constant flow pumps.

However, the benefit of providing a variable flow rate pump cannot beforgotten. While a constant flow of a medication such as a painkillermay indeed be useful in dulling chronic pain, it is very common forpatients to experience more intense pain. At times of this heightenedpain, it would be advantageous to be able to vary the flow rate of painkiller to provide for more relief. However, constant flow rate pumpstypically may only provide such relief by allowing for direct injectionsof painkillers or the like through the aforementioned bolus port, whichprovides direct access to the affirmed area. While indeed useful, thismethod amounts to nothing more than additional painful injections,something the pump is designed to circumvent.

Therefore, there exists a need for an implantable constant flow pump,which allows for a reduced overall size, as well as an implantable pumpthat combines the simplistic design of a constant flow rate type pumpand means for varying its flow rate, without requiring the use of thecomplex solutions provided by known programmable pumps.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a reduced size implantabledevice for dispensing an active substance to a patient. The implantabledevice of a first embodiment of this first aspect includes a housingdefining an active substance chamber in fluid communication with anoutlet for delivering the active substance to a target site within thepatient and a propellant chamber adjacent the active substance chamber.The implantable device further includes an undulating flexible membraneseparating the active substance and propellant chambers, wherein theactive substance chamber has an undulating surface including a centralconvex portion flanked by at least two concave portions, the undulatingsurface cooperating with the undulating flexible membrane.

In accordance with this first embodiment of the first aspect of thepresent invention, the propellant chamber may contain a propellantcapable of expanding isobarically where the propellant cooperates withthe flexible membrane to reduce the volume of the active substancechamber upon expansion of the propellant. The cooperating undulatingsurface of the active substance chamber and the undulating flexiblemembrane preferably meet upon complete expansion of the propellant. Theimplantable device may further include a replenishment opening in thehousing in fluid communication with the active substance chamber, and afirst septum sealing the opening. The replenishment opening may belocated within the central convex portion of the undulating surface ofthe active substance chamber so as to lower the overall height of thehousing of the implantable device. Additionally, the housing may includetwo portion beings constructed so as to screw together. The two portionsmay be constructed of PEEK. The two portions may be configured so as tocapture the membrane therebetween. Finally, the housing may also includea locking portion and/or a septum retaining member.

A second embodiment of this first aspect of the present invention is yetanother implantable device for dispensing an active substance to apatient. The implantable device according to this second embodimentincludes a housing defining a chamber and an outlet in fluidcommunication with the chamber for delivering the active substance to atarget site within the patient, the housing having a first portion and asecond portion, where the first and second portions are constructed ofPEEK and screwed together.

A third embodiment of this first aspect of the present invention is yetanother implantable device for dispensing an active substance to apatient. The implantable device according to this third embodimentincludes a housing including a top portion, a bottom portion and alocking portion. The housing defines a propellant chamber and an activesubstance chamber in fluid communication with an outlet. The implantabledevice preferably also includes a membrane retained between the top andbottom portions, the membrane separating the active substance andpropellant chambers. In a fully assembled stated, the top and bottomportions are preferably placed together and the locking portion engagesone of the top or bottom portions to retain the top and bottom portionstogether.

A fourth embodiment of this first aspect of the present inventionrelates to a method of assembling a reduced size implantable pump. Themethod of this embodiment includes the steps of placing together a topportion and a bottom portion to retain a membrane therebetween, andscrewing a locking portion into the top portion or the bottom portion toretain the top and bottom portions together.

A second aspect of the present invention includes an implantable devicefor dispensing an active substance to a patient including a housingdefining a chamber, said housing having an outlet for delivering theactive substance to a target site within the patient, the outlet influid communication with the chamber and means for varying the flow rateof the active substance between the chamber and the outlet. The chamber,in accordance with this second aspect of the present invention, mayinclude an active substance chamber in fluid communication with theoutlet and a propellant chamber, the active substance and propellantchambers being separated by a flexible membrane. The propellant chambermay contain a propellant capable of expanding isobarically andcooperating with the flexible membrane to reduce the volume of theactive substance chamber upon expansion of the propellant. The housingof the implantable device may include an opening in fluid communicationwith the active substance chamber and a first septum sealing theopening. The housing may further include an annular opening incommunication with the outlet and a second septum sealing the annularopening.

In a first embodiment of this second aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude an elongated polymer filament having a cross sectionaldimension. The filament, in accordance with this embodiment, ispreferably located in a capillary and is preferably capable of beingelongated to reduce the cross sectional dimension. In certain examples,the filament is located centrally within the capillary, in others, it islocated eccentrically. The filament may have a uniform cross section, asubstantially circular cross section, non-uniform cross section and thelike along its length. Further, this first embodiment may furtherinclude means for elongating the filament.

In a second embodiment of this second aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude a first hollow cylinder having a threaded exterior surface and asecond hollow cylinder having a threaded interior surface. The firsthollow cylinder is axially received within the second hollow cylinder,such that the threaded exterior surface of the first cylinder engagesthe threaded interior surface of the second cylinder. In thisembodiment, the axial movement of the first cylinder with respect to thesecond cylinder varies the flow rate of the active substance.

In a third embodiment of this second aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude a hollow tubular element having a cross section that is capableof being varied. This third embodiment may also include a capillary influid communication between the chamber and the outlet, where thetubular element is located therein. The hollow tubular element inaccordance with this embodiment may be centrally or eccentricallylocated within the capillary.

In a fourth embodiment of this second aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude an elongate insert having a longitudinally varying cross sectionalong its length. Movement of this elongate insert may increase ordecrease the flow rate of the active substance.

A third aspect of the present invention includes an implantable devicefor dispensing an active substance to a patient including a housingdefining a chamber, said housing having an outlet for delivering theactive substance to a target site within the patient, the outlet influid communication with the chamber. The implantable device alsoincludes a capillary in fluid communication between the chamber and theoutlet, the capillary having an inner surface and a flow control elementreceived within the capillary. The element has an outer surface opposingthe inner surface of the capillary defining therebetween a passagewayfor the flow of the active substance therethrough. The outer surface ofthe element is preferably movable relative to the inner surface of thecapillary to alter the flow of the active substance therethrough. Themovement of the outer surface of the element may alter the shape and/orsize of the passageway.

In a first embodiment of this third aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude an elongated polymer filament having a cross sectionaldimension. The filament, in accordance with this embodiment, ispreferably located in a capillary and is preferably capable of beingelongated to reduce the cross sectional dimension. In certain examples,the filament is located centrally within the capillary, in others, it islocated eccentrically. The filament may have a uniform cross section, asubstantially circular cross section, non-uniform cross section and thelike along its length. Further, this first embodiment may furtherinclude means for elongating the filament.

In a second embodiment of this third aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude a first hollow cylinder having a threaded exterior surface and asecond hollow cylinder having a threaded interior surface. The firsthollow cylinder is axially received within the second hollow cylinder,such that the threaded exterior surface of the first cylinder engagesthe threaded interior surface of the second cylinder. In thisembodiment, the axial movement of the first cylinder with respect to thesecond cylinder varies the flow rate of the active substance.

In a third embodiment of this third aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude a hollow tubular element having a cross section that is capableof being varied. This third embodiment may also include a capillary influid communication between the chamber and the outlet, where thetubular element is located therein. The hollow tubular element inaccordance with this embodiment may be centrally or eccentricallylocated within the capillary.

In a fourth embodiment of this third aspect, the means for varying theflow rate of the active substance between the chamber and the outlet mayinclude an elongate insert having a longitudinally varying cross sectionalong its length. Movement of this elongate insert may increase ordecrease the flow rate of the active substance.

A fourth aspect of the present invention includes a resistor for varyingthe flow rate of a fluid from a first point to a second point includinga capillary having an inner surface and a flow control element receivedwith the capillary. The element has an outer surface opposing the innersurface of the capillary such that a passageway is defined for the flowof fluid therethrough. The outer surface of the element is preferablymoveable relative to the inner surface of the capillary to alter theflow of the fluid therethrough. The movement of the outer surface of theelement may alter the shape and/or size of the passageway. It is notedthat this aspect may be utilized in conjunction with an implantabledevice such as an implantable pump for delivering a medicament to a sitewithin a patient. Embodiments in accordance with the third aspect areenvisioned that are similar to those discussed above in relation to thefirst and second aspects of the present invention.

A fifth aspect of the present invention includes a method of varying theflow rate of an active substance being dispensed to a patient. Thismethod includes the steps of providing an implantable device including acapillary having an inner surface and a flow control element receivedwithin the capillary. The element preferably has an outer surfaceopposing the inner surface of the capillary such that a passageway forthe flow of the active substance therethrough is defined therebetweenfor dispensing the active substance to a target site within a patient.Further the method includes the step of moving the element relative tothe inner surface of the capillary to alter the flow rate of the activesubstance therethrough. This moving step may alter the size and/or shapeof the passageway.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description in which reference ismade to the accompanying drawings in which:

FIG. 1 is a cross sectional front view of a reduced size implantablepump in accordance with one embodiment of the present invention.

FIG. 2 is a cross sectional bottom view of a portion of the reducedsized implantable pump shown in FIG. 1.

FIG. 3 is an enlarged view of an attachment area of the pump shown inFIG. 1.

FIG. 4 is a cross section front view of a reduced size implantable pumpin accordance with another embodiment of the present invention.

FIG. 5 is a cross section front view of a reduced size implantable pumpin accordance with another embodiment of the present invention.

FIG. 6 is a cross section front view of a reduced size implantable pumpin accordance with another embodiment of the present invention.

FIG. 7 is a cross sectional front view of an implantable constant flowpump for use in accordance with the present invention.

FIG. 8 is a cross sectional front view of another implantable constantflow pump for use in accordance with the present invention.

FIG. 9 is a cross sectional view of a variable flow resistor inaccordance with a first embodiment of the present invention having afilament located concentrically in a capillary.

FIG. 10 a is a longitudinal cross sectional view of the variable flowresistor of FIG. 9, in an initial position.

FIG. 10 b is a longitudinal cross sectional view of the variable flowresistor of FIG. 10 a, in an extended position.

FIG. 11 a is a cross sectional view of a variable flow resistor of thepresent invention having a filament located eccentrically in acapillary.

FIG. 11 b is a longitudinal cross sectional view of the variable flowresistor of FIG. 11 a, depicting the curvature of the capillary.

FIG. 12 a is a longitudinal cross sectional view of the variable flowresistor of FIG. 11 a, in an initial position.

FIG. 12 b is a longitudinal cross sectional view of the variable flowresistor of FIG. 12 a, in an extended position.

FIG. 13 is a longitudinal cross sectional view of another variable flowresistor in accordance with the present invention.

FIG. 14 is a longitudinal cross sectional view of another variable flowresistor in accordance with the present invention.

FIG. 15 is a cross sectional view of the driving assembly for use withthe flow resistor of FIG. 14.

FIG. 16 is a cross sectional view of a variable flow resistor inaccordance with a second embodiment of the present invention in a highresistance position.

FIG. 17 is a cross sectional view of the variable flow resistor of FIG.16 in a low resistance position.

FIG. 18 is a cross sectional view of a variable flow resistor inaccordance with a third embodiment of the present invention with aninsert centrally located.

FIG. 19 is a cross sectional view of a variable flow resistor inaccordance with a third embodiment of the present invention with aninsert eccentrically located.

FIG. 20 is a longitudinal cross sectional view of the variable flowresistor of FIG. 18.

FIG. 21 is a cross sectional view of the larger end of a variable flowresistor in accordance with a fourth embodiment of the present inventionwith an insert centrally located.

FIG. 22 is a cross sectional view of the larger end of a variable flowresistor in accordance with a fourth embodiment of the present inventionwith an insert eccentrically located.

FIG. 23 is a longitudinal cross sectional view of the variable flowresistor of FIG. 21.

FIG. 24 is a longitudinal cross sectional view of the variable flowresistor of FIG. 22.

FIG. 25 is a cross sectional view of a variable flow resistor inaccordance with a fifth embodiment of the present invention with aninsert centrally located.

FIG. 26 is a cross sectional view of a variable flow resistor inaccordance with a fifth embodiment of the present invention with aninsert eccentrically located.

FIG. 27 is a longitudinal cross sectional view of the variable flowresistor of FIG. 25.

FIG. 28 is a longitudinal cross sectional view of the variable flowresistor of FIG. 25.

FIG. 29 is a cross sectional view of an implantable pump in accordancewith another embodiment of the present invention.

FIG. 30 is a cross sectional view of the implantable pump shown in FIG.29, taken along a different portion thereof.

FIG. 31 is a partial top view of the implantable pump shown in FIG. 29.

DETAILED DESCRIPTION

In describing the preferred embodiments of the subject matterillustrated and to be described with respect to the drawings, specificterminology will be used for the sake of clarity. However, the inventionis not intended to be limited to any specific terms used herein, and itis to be understood that each specific term includes all technicalequivalents which operate in a similar manner to accomplish a similarpurpose.

Referring to the drawings, wherein like reference numerals refer to likeelements, there is shown in FIGS. 1 and 2, in accordance with variousembodiments of the present invention, a reduced size implantable pumpdesignated generally by reference numeral 1010. In a preferredembodiment, pump 1010 is a constant flow pump including a housing 1012,which further defines an interior having two chambers 1014 and 1016.Chambers 1014 and 1016 are preferably separated by a flexible membrane1018. It is noted that membrane 1018 may be of any design known in theart, for example, a membrane like that disclosed in commonly owned U.S.Pat. No. 5,814,019, the disclosure of which is hereby incorporated byreference herein. In a preferred embodiment, chamber 1014 is designedand configured to receive and house an active substance such as amedication fluid for the relief of pain, treatment of spasticity andneuro-mechanical deficiencies and the administration of chemotherapy,while chamber 1016 may contain a propellant that expands isobaricallyunder constant body heat. This expansion displaces member 1018 such thatthe medication fluid housed in chamber 1014 is dispensed into the bodyof the patient through an outlet catheter 1015 (best shown in FIG. 2).

The design and configuration of housing 1012 is such that manufacturingand assembly of pump 1010 is relatively easy. Housing 1012 furtherincludes separately manufactured top portion 1020, bottom portion 1022and locking portion 1024. It is noted that in certain preferredembodiments, housing 1012 defines a substantially circular pump 1010.However, the housing may ultimately be a pump of any shape. In additionto the above described elements, pump 1010 also preferably includesreplenishment port 1026 covered by a first septum 1028 that is in fluidcommunication with chamber 1014 through a channel 1029, an annular ringbolus port 1030 covered by a second septum 1032, and barium filledsilicone o-ring 1033. Each of these elements will be discussed furtherbelow.

Referring to both FIGS. 1 and 2, where FIG. 2 is a cross sectionalbottom view of locking portion 1024, the flow path of a medication fluidcontained within chamber 1014 is shown. Upon the expansion of propellantcontained within propellant chamber 1016 and the necessary displacementof membrane 1018, fluid contained in chamber 1014 is forced through anopening 1049 and into a cavity 1046, which will be further describedbelow. As shown in FIG. 2, cavity 1046 extends in a circular fashionaround pump 1010. Once in cavity 1046, the fluid may enter at any pointalong the length of a filter capillary 1072. Essentially, filtercapillary 1072 is a well known type filter that allows for fluid toenter into its inner fluid path through permutation or the like. Thus,once a certain amount of fluid builds up within cavity 1046, it iscapable of entering into filter 1072. This filter is preferably fixedand sealed in position by drops of glue or other adhesive located at1070 and 1074. The fluid then travels through filter capillary 1072until it exits into a resistor 1076. This resistor is preferably a LONGtube having a relatively small diameter, so as to dictate the maximumflow rate that may be achieved therethrough. In other words, the smallerthe diameter of resistor 1076, the slower the flow rate of fluidtraveling therethrough. Nevertheless, as more fully discussed below,resistor 1076 may be many different types of designs. The fluid withinresistor 1076 then continues to an opening 1078 for a bridge 1080, whichessentially allows resistor 1076 to cross over bolus port 1030.Thereafter, the fluid may continue through resistor 1076 and ultimatelyout catheter 1015. Epoxy or another suitable adhesive or sealant may beutilized to seal end 1070, end 1074 and opening 1078. Thus, fluid incavity 1046 may only follow the path outlined above.

It is noted that FIG. 2 also depicts the flow path that fluid introducedthrough a bolus injection may take. Fluid may be injected into bolusport 1030 through the use of a device suitable for piercing septum 1032,such as a needle. Once in port 1030, which extends around pump 1010,fluid may enter a channel 1082. This channel extends at least partiallyaround the above mentioned bridge 1080, and allows fluid injected intobolus port 1030 to ultimately exit catheter 1015 without passing throughany portion of resistor 1076. As shown in FIG. 2, regardless of the paththe fluid takes, it ultimately ends up in a passage 1084 just prior tocatheter 1015. Thus, fluid coming from chamber 1014 may have one flowrate, while fluid directly injected into port 1030 may have a differentflow rate, the latter preferably being greater.

The assembly of pump 1010 will now be discussed. It is noted that eachof the individual elements/components of pump 1010 may be individuallymanufactured and thereafter assembled by hand or by another process,such as an automated process. As an initial step, top portion 1020 andbottom portion 1022 are placed or sandwiched together so as to capturemembrane 1018 therebetween in an attachment area 1034 for fixablyretaining same. As more clearly shown in the enlarged view of FIG. 3,attachment area 1034 comprises a projection 1036 located on bottomportion 1022, a depression 1038 located on top portion 1020, and acavity 1040 formed through the cooperation of the two portions. Inoperation, the step of sandwiching together portions 1020 and 1022, withmembrane 1018 disposed therebetween, causes projection 1036 to be forcedinto depression 1038. The portion of membrane 1018 disposed therebetweenis thus also forced into depression 1038 by projection 1036. This causesa crimp-like connection, which fixably attaches and retains membrane1018 within housing 1012. As shown in FIG. 3, membrane 1018 may consistof multiple layers, of which all are preferably “crimped” during theattachment process. Prior to pressing together portions 1020 and 1022, alayer of epoxy or other adhesive may be inserted into cavity 1040. Insuch embodiments that employ the use of an adhesive, the design maycause portions 1020 and 1022 to become fixably attached to one anotherupon the sandwiching of same. Further, the use of an adhesive withincavity 1040 may also aid in the fixation of membrane 1018 between thetwo portions. The epoxy or other adhesive may be placed into the cavityportion formed on either portion 1020 or portion 1022, prior to thesandwiching step.

Prior or subsequent to the assembly of top portion 1020 together withbottom portion 1022, o-ring 1033 or the like may be placed into aring-shaped cavity formed in top portion 1022. In certain preferredembodiments, o-ring 1033 is a barium filled silicone o-ring, and isdisposed around the area defining replenishment port 1026. Such ano-ring design allows for the area defining replenishment port 1026 to beilluminated under certain scanning processes, such as X-rays. As pump1010 is implanted within the human body, locating port 1026, in order torefill the pump with medicament or the like, may be difficult. Providinga barium filled o-ring 1033, which essentially outlines the area of port1026, allows for a doctor to easily locate the desired area under wellknown scanning processes. Other structures may be utilized, in whichsame also show up on different scans. The placement of o-ring 1033 ispreferably accomplished by pressing the o-ring into an undersizedchannel that retains the o-ring, thereafter.

With o-ring 1033 preferably in place, locking portion 1024 is nextattached to the other portions. It is noted that prior to attachingportion 1024, first septum 1028 should be inserted into locking portion1024. Preferably, first septum 1028 is slid into a complimentary cavityformed in portion 1024, such that it remains within absent a forceacting upon same. As first septum 1028 is designed to be capturedbetween locking portion 1024 and top portion 1020, the septum should beplaced prior to the attachment of locking portion 1024. In addition, asmentioned above, locking portion 1024 may include a second septum 1032for covering bolus port 1030. In certain preferred embodiments, as shownin FIG. 1, second septum 1032 is ring shaped, and is pressed intolocking portion 1024 in a similar fashion to that discussed above withrelation to the placement of o-ring 1033. This may be done prior orsubsequent to the attachment of locking portion 1024 to the otherportions.

With regard to the attachment step, locking portion 1024 preferablyincludes a threaded area 1042 for cooperating with a threaded extension1044. In operation, locking portion 1024 is merely screwed intoengagement with bottom portion 1022. This necessarily causes top portion1020, which is disposed between the two other portions, to be retainedtherebetween. In other words, the screw attachment of locking portion1024 with bottom portion 1022 not only causes such portions to befixably attached to one another, but also causes top portion 1020 to befixably retained therebetween. It is noted that, depending upon howtight locking portion 1024 is screwed into 1022, portions 1020 and 1022may be further pressed together, thereby increasing the fixation ofmembrane 1018 therebetween. Thus, pump 1010 is designed so that minimalconnection steps are performed in order to cause all of the componentsthereof to be retained together. It is further noted that, in additionto the above discussed screw connection of portions 1022 and 1024, otherattachment means may be utilized. For example, such portions may be snapfit together or fixed utilizing an adhesive. Finally, locking portion1024 may be configured so as to form cavity 1046 between itself and topportion 1020. This cavity may be designed so as to allow for theinjection of adhesive therein, thus increasing the level of fixationbetween the different portions of housing 1012. Additionally, cavity1046 may house a flow resistor or the like, as will be more fullydiscussed below.

As set forth above, pump 1010 is configured and dimensioned to berelatively simplistic in both manufacture and assembly. However, pump1010 is also configured and dimensioned so as to employ a significantlyreduced overall size, while still providing for a useful amount ofmedicament and propellant to be housed therein. In the preferredembodiments depicted in the figures, top portion 1020 of pump 1010includes an interior surface 1047 having an undulating or convolutedshape. More particularly, surface 1047 includes a convex central portionflanked by two concave portions. This configuration allows for thecentrally located replenishment port 1026 and cooperating septum 1028 tobe situated in a lower position with respect to the remainder of pump1010. At the same time, the aforementioned flanking concave portionsallow for the overall volume of chambers 1014 and 1016 to remainsubstantially the same as a pump employing an interior surface havingone constant concave portion or the like. In other words, the flankingconcave portions make up for the volume lost in situating port 1026 andcooperating septum 1028 in a lower position. Membrane 1018 is alsopreferably configured so as to have an initial undulating shape forcooperation with interior surface 1047. Thus, with no medicament orother fluid located within chamber 1014, membrane 1018 preferably restsagainst surface 1047. However, upon injection of fluid into chamber1014, membrane 1018 adapts to the position shown in FIG. 1.

FIG. 4 depicts another reduced sized implantable pump designated byreference numeral 1110. As shown in the figure, pump 1110 includesseveral elements which are similar in structure and function to that ofpump 1010. These elements are labeled with like references numeralswithin the 1100 series of numbers. For example, membrane 1118 is similarto the above described membrane 1018. In addition, pump 1110 operates ina similar fashion to that of pump 1010. Nevertheless, pump 1110 doesinclude certain additional elements, as well as elements employingdifferent constructions. Most notably, pump 1110 includes an additionalcomponent, namely septum retaining member 1125. This member ispreferably adapted to be screwed into top portion 1120. Pump 1110 alsoincludes a bottom o-ring 1150, but does not include a barium filledo-ring.

The assembly of pump 1110 also differs from that of pump 1010. Asbriefly mentioned above, initially, septum retaining member 1125 isfirst screwed into top portion 1120 in order to retain previously placedseptum 1128 in place. Like the above described assembly of pump 1010,the assembly of pump 1110 then includes the step of sandwiching togetherportions 1120 and 1122, where membrane 1118 is likewise capturedtherebetween in attachment area 1134. However, in this embodiment,locking portion 1124 is adapted to engage top portion 1120, so that itis positioned on the bottom side of pump 1110. As shown in FIG. 4, topportion 1120 includes a threaded extension 1152 to cooperate and engagewith threaded area 1142 of locking portion 1124. The screw connectionbetween the two portions is similarly achieved. However, bottom o-ring1150 is preferably situated between locking portion 1124 and bottomportion 1122. This o-ring both increases the force exerted on bottomportion 1122 by locking portion 1124, and also causes housing 1112 toretain a smooth exterior surface. The latter is important in implantingthe pump within a patient, as rough or jagged surfaces may cause damageto tissue abutting the pump. Finally, it is noted that second septum1132 may be pressed into top portion 1120, at any point during theassembly.

FIG. 5 depicts another reduced sized implantable pump designated byreference numeral 1210. As shown in that figure, pump 1210 includesseveral elements which are similar in structure and function to that ofpumps 1010 and 1110. Once again, these elements are labeled with likereference numerals within the 1200 series of numbers. Nevertheless, pump1210 does include certain additional elements, as well as elementsemploying different constructions. For example, like pump 1110, pump1210 includes a septum retaining member 1225. Similarly, like pump 1010,pump 1210 utilizes a top mounting locking portion 1224, although it hasa different construction.

The assembly of pump 1210 differs from that of the above discussed pumps1010 and 1110. Like pump 1110, septum retaining member 1225 is firstscrewed into top portion 1220, in order to retain previously placedseptum 1228 in place. Next, portions 1120 and 1222 are sandwichedtogether, thus capturing member 1218 within attachment 1234. Finally,locking portion 1224 is screwed into engagement with bottom portion1222. Like the design of pump 1010, locking portion 1224 includes athreaded area 1242 which engages a threaded extension 1244 of bottomportion 1222. In addition to completing the assembly of pump 1210 bycapturing bottom portion 1222 and forcing top portion 1220 towardsbottom portion 1222, locking portion 1224 is configured and dimensionedin this embodiment to also capture second septum 1232. As shown in FIG.5, locking portion 1224 includes a concave section 1254 for engagingseptum 1232 upon the full engagement of portions 1222 and 1224.

Yet another embodiment reduced sized pump 1310 is shown in FIG. 6. Likethose pumps discussed above, pump 1310 preferably includes severalelements which are similar in structure and function, and are thuslabeled with like reference numerals within the 1300 series of numbers.Essentially, pump 1310 is akin to the configuration set forth in pump1210. However, there are two main distinctions, namely, the cooperationof locking portion 1324 and portions 1320 and 1322, and the inclusion ofa channel 1362 between locking portion 1324 and top portion 1320. In theembodiment depicted in FIG. 6, it is noted that locking portion 1324includes a threaded extension 1356, which cooperate and engage threadedareas 1358 and 1360 of portions 1320 and 1322, respectively.Furthermore, locking portion 1324 preferably includes a channel 1362formed therein. This channel may be adapted to cooperate with any of thechambers and/or ports discussed above. Additionally, channel 1362 mayhouse other elements, such as a flow resistor or the like, which will bediscussed more fully below.

A second aspect of the present invention relates to providing a constantflow type implantable pump with infinitely variable flow capabilities. Amentioned above, such a construction may be beneficial to patientsrequiring more or less medication to be delivered by an implantablepump. While the different embodiments of this second aspect of thepresent invention may indeed be sized and configured to be utilized withany constant flow type implantable pump, preferred pumps will bedescribed herein. In one preferred pump, as shown in FIG. 6 of thepresent application, the basic implantable pump design is designated asreference numeral 20. Pump 20 includes a housing 22 defining an interiorhaving two chambers 24 and 26. Chambers 24 and 26 are separated by aflexible membrane 28. Chamber 24 is designed to receive and house theactive substance such as a medication fluid for the relief of pain,treatment of spasticity and neuro-mechanical deficiencies and theadministration of chemotherapy, while chamber 26 may contain apropellant that expands isobarically under constant body heat. Thisexpansion displaces membrane 28 such that the medication fluid housed inchamber 24 is dispensed into the body of the patient through the pathdefined by an outlet opening 30, a resistor 32, an outlet duct 34 andultimately an outlet catheter 36.

Resistor 32 provides a connection between chamber 24 and outlet duct 34.Thus, as mentioned above, a medication fluid flowing from chamber 24 tooutlet catheter 36 must necessarily pass through resistor 32. Thisresistor allows for the control of the flow rate of the medicationfluid, such that the flow rate is capable of being varied. Resistor 32may be configured differently in many different embodiments, some ofwhich are discussed below in the detailed description of the presentinvention. Essentially, resistor 32 defines a passageway for the flow ofthe medication fluid, where the passageway may be altered to therebyalter the flow rate of the medication fluid.

Implantable pump 20 also includes a replenishment port 38 covered by afirst septum 40. Septum 40 can be pierced by an injection needle (suchas needle 42 shown in FIG. 7) and, upon removal of such needle, iscapable of automatically resealing itself. Septa of this type are wellknown to those of ordinary skill in the art. As implantable pump 20 isdesigned to medicate a patient over a limited period of time,replenishment port 38 is utilized for replenishing chamber 24 when emptyor near empty. In operation, a physician or other medical professionalinserts an injection needle 42 into an area of a patient's body wherepump 20 is located, such that it may pierce septum 40. Thereafter,operation of the needle causes injection of the solution from the needleto pass into port 38, through passage 44, and into chamber 24. It isnoted that the particular dimension and/or the patient's need mayrequire such a process to be repeated at given intervals, for example,monthly, weekly, etc.

In addition to replenishment port 38, pump 20 also includes an annularring bolus port 46 covered by a second septum 48. Essentially, this portallows for direct introduction of a solution into outlet catheter 36 andto the specific target area of the body. This port is particularlyuseful when a patient requires additional or stronger medication, suchas a single bolus injection, and/or when it is desired to test the flowpath of catheter 36. Such an injection is performed in a similar fashionto the above discussed injection into replenishment port 38. However, aninjection into bolus port 46 bypasses passage 44, chamber 24 andresistor 32, and provides direct access to catheter 36. It is alsocontemplated to utilize bolus port 46 to withdraw fluid from the body.For example, where pump 20 is situated within the body such thatcatheter 36 extends to the vertebral portion of the spinal column, aneedle with a syringe connected may be inserted into bolus portion 46and operated to pull spinal fluid through catheter 36 and into thesyringe.

In certain embodiments, septum 40 and septum 48 may be situated so thatonly specifically designed injection needles may be used to inject intothe respective ports. For example, as is also shown in FIG. 7, septum 48may be situated relatively close to the bottom of port 46 and septum 40may be situated a greater distance away from the bottom of port 38. Inthis embodiment, injection needle 42 is provided with an injection eye43, which is located above the tip of needle 42. Alternatively,injection needle 50 is provided with an injection eye 51 located at ornear its tip. This arrangement prevents needle 42, which is typicallyutilized for replenishing chamber 24 with a long term supply ofmedication fluid, from being inadvertently used to inject its contentsinto bolus port 46. As is shown on the left side depiction of bolus port46, needle 42 would have its eye 43 blocked by septum 48 if the needleis inadvertently inserted into this port. Needle 50, on the other hand,would be capable of injecting into port 46 because of the lower locationof its eye 51. This is an important safety feature, as direct injectionof a long term supply of medication fluid into port 46 could bedangerous. It is noted that needle 50 is also capable of injecting asolution into replenishment port 38, however, the same concerns(i.e.—over-medication) do not exist with respect to the filling ofchamber 24, and as such medication housed in the chamber is slowlyreleased. While this is one example of a possible safety feature withregard to the injection of materials into the pump, it is envisionedthat other safety precautions may be utilized. For example, U.S. Pat.No. 5,575,770, the disclosure of which is hereby incorporated byreference herein, teaches a similar multiple injection needle systemwith additional valve protection. It is noted that such a safety needlesystem may be employed with regard to any of the various implantablepump embodiments disclosed herein. One of ordinary skill in the artwould recognize the modifications required to utilize such a safetyfeature in the other discussed pump designs.

In other embodiments, the basic implantable pump design of theaforementioned '873 patent may also be utilized. As is discussed in itsspecification and shown in FIG. 8 of the present application, the '873patent discloses a housing made up of two parts 1, 2 and an interiorhaving two chambers 4, 5, which are separated by a flexible membrane 3.Chamber 4 is designed to receive and house the medication fluid, whilechamber 5 may contain a propellant which, like that discussed in theabove description of pump 20, expands isobarically under constant bodyheat. This expansion displaces membrane 3 such that the medication fluidhoused in chamber 4 is dispensed into the body of the patient throughthe path defined by an outlet opening 6, an outlet reducing means 7 andultimately an outlet catheter 8. It is noted that reducing means 7 ispreferably a tube winding that wraps around part 1 of the housing. Theresistor of the present invention, in certain embodiments, is preferablylocated at or near outlet opening 6. This will be discussed more fullybelow.

Prior to reaching outlet catheter 8, the medication fluid is introducedinto a chamber 9 which is provided annularly on part 1 of the housing.Chamber 9 is sealed at its upper side by a ring or septum 10, which canbe pierced by an injection needle and which automatically reseals uponwithdrawal of the needle. This chamber is similar to the above discussedbolus port 46 of pump 20. In addition to allowing medication fluid fromchamber 4 to pass into outlet catheter 8, chamber 9 also allows thedirect injection of a solution into outlet catheter 8, the importance ofwhich is discussed above. The aforementioned outlet reducing means 7prevents a solution injected into the bolus port from flowing intochamber 4. In a similar fashion, when need be, chamber 4 may bereplenished via a further septum 12. Once again an injection needle maybe utilized for this purpose.

While two basic designs of implantable pumps are described above, it isnoted that other designs may include different or additional elements.Similarly, while the above description teaches two implantable pumpsthat may be utilized in accordance with the present invention, otherimplantable pump designs are also capable of being utilized. Forexample, U.S. Pat. Nos. 5,085,656, 5,336,194, 5,722,957, 5,814,019,5,766,150, 5,836,915 and 6,730,060, the disclosures of which are allhereby incorporated by reference herein, may be employed in accordancewith the present invention. In addition, one specific embodiment will bediscussed below.

As mentioned above, the capability of varying the flow rate of animplantable pump is desired. In the above discussed constant flow pumps,the flow rate of the medication fluid depends upon the pump pressure,the pressure at the end of the catheter and the hydraulic resistance ofany of the capillaries or other passages that the medication fluid musttravel through. With regard to the resistance of the capillaries, suchresistance depends upon the geometry of the capillary itself, as well asthe viscosity of the medication fluid. This viscosity, as well as thepump pressure, may both be influenced by body temperature. As such, oneinstance in which it is desired to control the flow rate of the pumpexists if the patient develops a fever because the flow rate of theinfusion device may be affected in an undesired way.

Another example of when the variable flow rate of the implantable pumpis desired relates to the condition or active status of the patient. Forexample, especially in the case where painkillers are beingadministered, it may be advantageous to deliver less medication duringthe nighttime hours, when the patient is sleeping. Additionally, asdiscussed above, it may be desirable to be able to increase the dosageof such painkillers or the like when the patient's symptoms worsen.Increasing of the flow rate of the medication fluid may be necessary inorder to diminish the patient's pain level. In accordance with thepresent invention, the aforementioned resistor 32 is useful foradjusting the flow rate in order to counteract undesirable flow ratechanges due to body temperature changes, and to allow for desiredadjustments of flow rate to treat heightened or worsened symptoms.

In a first embodiment this adjustment of flow rate is realized byadjusting the cross-sectional geometry of an article of the resistor. Itis noted that the first embodiment will be discussed with respect topump 20; however, it may be utilized in combination with any implantablepump. As shown in FIGS. 9-15, in accordance with this first embodiment,resistor 32 includes an elastic and resilient filament 52 situated in aresistor capillary 54, where resistor capillary 54 provides a connectionbetween outlet opening 30 and outlet capillary 34. Capillary 54 may besituated so as to constitute substantially the entire outlet capillary34, or may only be a portion thereof. Essentially, capillary 54 needonly require the aforementioned medication fluid to pass therethrough,and thus, may be any length suitable for use in varying the flow rate.

FIGS. 9, 10 a and 10 b show a first example of the first embodimentresistor 32, where elastic filament 52 is located concentrically inresistor capillary 54. This configuration forms a ring-shaped flowchannel 56 through which fluid flows in a direction shown by arrow F. Asis best shown in FIG. 10 a, filament 52 includes a first end 58 attachedto a stationary attachment 60, and a second end 62 attached to a movableattachment 64. Resistor 32 also has an effective length L extendingbetween capillary entrance 66 to exit 68, and an initial diameter D1(i.e.—2 times its radius R1). Additionally, capillary 54 has a diameterD3 (i.e.—2 times its radius R3). This will be similar throughout in thevarious other capillaries discussed herein.

In this example, movable attachment 64 is capable of moving in theopposite longitudinal directions shown by arrows A and B, whileattachment 60 remains stationary. In operation, movement of attachment64 in the direction of arrow B increases the distance betweenattachments 62 and 64 and also results in the decrease of the initialdiameter D1 to a lesser diameter D2 (i.e.—2 times its lesser radius R2).This is best shown in FIG. 10 b. The decrease of the diameter offilament 52 from D1 to D2 increases the size of channel 56 and thusnecessarily decreases the hydraulic resistance in capillary 54.Oppositely, movement of attachment 64 in the direction of arrow Areturns filament 52 to the position shown in FIG. 10 a, and increasesthe hydraulic resistance in capillary 54. A filament of this type may beconstructed of silicone rubber, or other suitable polymer materials forproviding the required elasticity and resiliency so as to return to itsoriginal shape and size after being deformed by stretching. Similarly,although filament 52 is shown in the figures as having a substantiallycircular cross section, it is envisioned that filaments having othercross sections may be utilized, for example, polygonal, oval, square andthe like.

As the inner diameter of capillary 54 is typically very small (on theorder of several thousands of millimeters), it is often difficult tolocate filament 52 directly in the center of the capillary. FIGS. 11 a,11 b, 12 a and 12 b depict a second example where elastic filament 52touches the inner wall of capillary 54 (i.e.—an eccentric position).This eccentrically placed filament 52 creates a sickle-shaped flowchannel 56, as opposed to the ring-shaped flow channel of the firstexample. This second example also differs from the first examplediscussed above, in that both ends 58, 62 of filament 52 are attached tomovable attachments 60, 64, respectively. This is useful, as inoperation, one movable attachment (or the mechanism moving it) may fail.The two movable attachment design provides a failsafe, thereby allowingfilament 52 to be stretched through the movement of the non-failingattachment. Attachment 64 is still capable of moving in the directiondepicted by arrows A and B and attachment 60 is capable of moving in thedirection depicted by arrows A′ and B′.

In operation, movement of either of attachments 60, 64 in the directionsB′ and B, respectively, decreases the diameter D1 to a lesser diameterD2 (once again, these diameters refer to two times the radii R1 and R2,respectively). This position is best shown in FIG. 12 b. Like that ofthe above discussed first example, this decrease in the diameter offilament 52 from D1 to D2 increases the size of channel 56 and thusnecessarily decreases the hydraulic resistance in capillary 54.Oppositely, movement of either of attachments 60, 64 in the direction ofarrows A′ and A, respectively, returns filament 52 to the position shownin FIG. 12 a, and increases the hydraulic resistance in capillary 54.

Attachment 64 in the first example, and attachments 60, 64 in the secondexample may be moved by any means known to those of ordinary skill inthe art. For example, it is well known to utilize micro-motors, magnets,or other hydraulic, electrical or mechanical actuators. One example of asuitable motor assembly is sold under the designation X15G by ElliptecResonant Actuator of Dortmund, Germany.

In accordance with the present invention, it is known to design acapillary with a circular lumen defined by a rigid wall. Essentially,this type of apparatus is a hollow tube having a flow therethrough(i.e.—the present design without filament 52). For such a design, theflow rate can be calculated using the well-known Hagen-PoisseuilleEquation:V=(ΔpπR ₂ ⁴)/(8ηL)

Where:

V=flow rate

Δp=pressure difference between entrance 66 and exit 68 of capillary 54.

η=viscosity of fluid.

L=effective length L of resistor 32.

R₂=radius of resistor capillary 54 (see in FIG. 9).

As shown in the above equation, small changes in the diameter of acapillary have a profound effect on the flow rate. However, themodification of the R₂ dimension is often technically very difficult torealize. Thus, as discussed above, the design of this first embodimentof the present invention includes implementing elastic filament 52 intoresistor capillary 54, as discussed above. For the first example of thefirst embodiment (i.e.—concentrically located filament 52), thefollowing equation may be utilized in determining the flow rate of thisdesign:V=[(Δpπ)(R ₂ −R ₁)³(R ₂ +R ₁)]/(8ηL)

Where:

V=flow rate

Δp=pressure difference between entrance 66 and exit 68 of capillary 54.

η=viscosity of fluid.

L=effective length L of resistor 32.

R₁=radius of filament 52 (see in FIG. 9).

R₂=radius of resistor capillary 54 (see in FIG. 9).

Alternatively, for the second example of the first embodiment(i.e.—eccentrically located filament 52), the following equation may beutilized in determining the flow rate of this design:V=[(Δpπ)(R ₂ −R ₁)³(R ₂ +R ₁)2.5]/(8ηL)

Where:

V=flow rate

Δp=pressure difference between entrance 66 and exit 68 of capillary 54.

η=viscosity of fluid.

L=effective length L of resistor 32.

R₁=radius of filament 52 (see in FIG. 9).

R₂=radius of resistor capillary 54 (see in FIG. 9).

All three of the above equations are well known in the field of fluiddynamics. Further, while the effective length L of resistor 32, as bestshown in FIGS. 10 a and 12 a, corresponds to the length of capillary 54,it is noted that the effective length more specifically relates to thelength of capillary 54 in which filament 52 resides. Therefore, theeffective length L, for use in the above equations, may be less than thelength of capillary 54 if filament 52 has a length less than the lengthof capillary 54. It is noted that these equations apply to the use ofcapillaries and filaments having circular cross sections. Otherembodiments may utilize differently shaped capillaries and filaments.For these embodiments, separate equations must be utilized.

As is clearly shown by the second equation, situating filament 52 in theoffset position with relation to the center of capillary 54 of, as shownin FIG. 11 a, allows the flow rate to be changed by a factor of 2.5.Therefore, for applications where it is desired to vary the flow rate bysuch a ratio, it is possible to merely move filament 52 from a centralposition taught in the first example (as shown in FIG. 9) to theeccentric position taught in the second example (as shown in FIG. 11 a).However, often times, it is typically desired to vary the flow rate by afactor of 25 or more. In order to achieve such a flow rate change, onemay utilize an elastic filament 52 as discussed above, situated in anoffset position. Typically, to ensure that filament 52 remains in theoffset position, a curved capillary 54 is utilized. As shown in FIG. 11b, filament 52 remains eccentrically placed within capillary 54 becauseof the curvature of the capillary. As filament 52 is generally elasticand resilient, it easily conforms to any curvature of capillary 54.

A realistic range for the change in diameter of elastic filament 52 isapproximately from its original size to about seventy percent of itsoriginal size (i.e.—a 1 to 0.7 ratio). Calculations have been carriedout using the above equation relating to the eccentrically positionedfilament 52. For example, with the initial radius R1 of filament 52being approximately eighty percent (80%) of the radius R2 of capillary54 (i.e.—a 0.8 to 1 ratio) and the maximal elongation of filament 52giving a radius R3 that is approximately fifty six percent (56%) of theradius R2 of capillary 54 (i.e.—a 0.56 to 1 ratio), it was calculatedthe ratio of flow rate between the non-elongated state and the maximalelongated state is approximately 9.20 to 1. With the initial radius R1of filament 52 being approximately eighty five percent (85%) of theradius R2 of capillary 54 (i.e.—a 0.85 to 1 ratio) and the maximalelongation of filament 52 giving a radius R3 that is approximately fiftynine point five percent (59.5%) of the radius R2 of capillary 54 (i.e.—a0.595 to 1 ratio), it was calculated the ratio of flow rate between thenon-elongated state and the maximal elongated state is approximately17.00 to 1. Finally, with the initial radius R1 of filament 52 beingapproximately ninety percent (90%) of the radius R2 of capillary 54(i.e.—a 0.9 to 1 ratio) and the maximal elongation of filament 52 givinga radius R3 that is approximately sixty three percent (63%) of theradius R2 of capillary 54 (i.e.—a 0.63 to 1 ratio), it was calculatedthe ratio of flow rate between the non-elongated state and the maximalelongated state is approximately 43.46 to 1. Thus, using a filament 52having a radius R1 between approximately eighty five percent (85%) andninety percent (90%) of the total radius R2 of capillary 54, wouldresult in a flow rate variation of approximately 25. From the foregoing,one can calculate the desired flow rate variation based on the knowngeometry of the flow resistor.

A third example of the first embodiment of the present invention isshown in FIG. 13. This example includes a capillary 154 that is dividedinto two sectors by a center wall 155. Fluid is capable of flowingthrough capillary 154 by entering through entrance 166 and exitingthrough exit 168, as depicted by fluid flow arrow F. An elastic filament152 is fixed at its ends by fixation points 160 and 164, and is wrappedaround a magnetic element 170 at the approximate central portion offilament 152. Repulsive magnetic forces are transmitted to magneticelement 170 by a corresponding magnetic counterpart 172, having asimilar polarity. Thus, movement of counterpart 172 results in the likemovement of element 170. Counterpart 172 may be located in ahermetically sealed housing 174, or the like. Movement of the magneticelement in a direction indicated by arrow B will, as in the abovediscussed examples, cause the diameter of filament 152 to shrink,thereby allowing for the increase in flow rate. Similarly, movement ofelement 170 in the direction indicated by arrow A will decrease the flowrate. It is noted that this two sector design includes two capillary andfilament relationships for use in varying the flow rate. As such, whereboth the capillary and the filament have circular cross sections, twoseparate calculations in accordance with the above discussed equations,must be conducted to determine the overall hydraulic resistance providedby the system.

Further, in accordance with this third example of the first embodiment,it is envisioned that magnetic element 170 and magnetic counterpart 172may be oppositely polarized, such that they are attracted to oneanother. In this type of design, moving counterpart 172 in a directioncloser to element 170 would cause the attraction between them to begreater. Thus, if counterpart 172 is located below element 170 (asopposed to that shown in FIG. 13), movement of counterpart 172 towardselement 170 would increase the magnetic attractive force between the twocomponents and necessarily cause the movement of element 170 in thedirection indicated by arrow B. As discussed above, this lengthensfilament 152, while at the same time decreasing its diameter. Thus, thiswould constitute one alternate design. Similarly, it is possible toprovide a single magnetic component with a corresponding metalliccomponent, rather than the above discussed two magnet configuration.Clearly, as is well understood, such components would be attracted toone another. Therefore, operation of this magnet/metal configurationwould operate in a like manner to the above discussed opposite polaritymagnetic configuration. However, it is to be understood that variousconfigurations are envisioned depending upon the polarity of themagnetic components and/or the situation of the metallic element and itscorresponding magnetic element. For example, filament 152 may be wrappedaround a metallic element, with a magnetic component located in housing174 or vice versa.

A fourth example of the first embodiment of the present invention isshown in FIG. 14. This example includes an elastic filament 252 that isfixed at one end by attachment 260 and wrapped around axle 276 on theother. Once again, fluid enters capillary 254 at entrance 266, and exitsat exit 268. Fluid flow direction is once again indicated by arrow F.Rotation of axle 276, in a direction depicted by arrow W(i.e.—counter-clockwise), causes filament 252 to lengthen, while itsdiameter reduces. This, in turn, increases the possible flow ratethrough capillary 254. Alternatively, rotation of axle 276 in aclockwise direction causes the opposite effect. As previously mentioned,if filament 252 and filament 254 have circular cross sections, the aboveequations may be utilized in calculating the hydraulic resistance of thesystem. Axle 276 may be driven directly by a micro motor, via areduction gear drive assembly 280 as shown in FIG. 15.

While other means may be utilized for driving axle 276, the followingsets forth a discussion of the aforementioned reduction gear driveassembly 280. As shown in FIG. 15, assembly 280 presents a solution forthe transfer of rotational motion from hermetic enclosure 274 to axle276. Assembly 280 includes a motor 282 that is augmented by a gear drive284 and transferred to disc 286. The disc includes a shaft 288 which ispreferably positioned at an angle which is less than ninety degreerelative to the plane of disc 286. Shaft 288 extends into cylindricalportion 290 of hermetic enclosure 274. Further, shaft 288 is supportedvia bearings 292 within cylindrical portion 290. Finally, cylindricalportion 290 is connected to enclosure 274 by an elastic connection 294and is capable of transmitting forces via pusher plate 296 to rotateaxle 276. Essentially, the offset nature of the connections between disc286 and shaft 288, and portion 290 and plate 296, coupled with theelastic nature of the connection between enclosure 274 and portion 290allows for the rotation of axle 276. It is noted that operation of themotor in different directions causes the rotation of the axle in theclockwise or counter-clockwise direction.

Gear drive assembly 280 is useful for allowing a relatively small orweak motor to drive axle 276. Providing a gear assembly to betterutilize a motor is well known. However, any known gear assembly,suitable for use with the present invention, may be employed. Further,it is also contemplated that a suitable motor may be employed that maybe capable of directly rotating axle 276. Essentially, in a design likethis, axle 276 may be a continuation of the drive shaft of the motor.

Any of the examples set forth in the discussion relating to this firstembodiment may include different, additional or fewer elements. Suchrevisions will be understood by those of ordinary skill in the art. Forexample, it is envisioned that the various elastic filaments, whileshown in the figures having a substantially circular cross section, mayinclude any shaped cross section. Similarly, although shown assubstantially straight, the above may be utilized in conjunction withcurved capillaries. Additionally, it is to be understood that theinventions set forth in the first embodiment may be utilized with anyknown implantable pump. The particular pump design may require the useof a resistor that is particularly configured and dimensioned to operatewith the pump. Such design requirements are evident to those of ordinaryskill in the art.

In a second embodiment the adjustment of flow rate is realized byproviding a pair of threaded matched cylinders for use as resistor 32.Once again, the second embodiment will be discussed with respect to pump20; however, it may be utilized in combination with any implantablepump. As shown in FIGS. 16 and 17, in accordance with this secondembodiment, resistor 32 includes a first threaded member 302 having ahollow interior 304 and a threaded exterior 306. First threaded memberis disposed in second threaded member 308, which is an oppositelyconfigured hollow member having a threaded interior surface 310 and aclosed end 312. The threaded cooperation between first and secondthreaded members 302 and 308 allows for the first member to be disposedwithin the second member at varying levels, therefore, allowing fordifferent overlaps of the two members. For example, FIG. 16 depicts thefirst member being substantially disposed within the second member,while FIG. 17 depicts the first member being only partially disposedwithin the second member.

In operation of this second embodiment, fluid is introduced into hollowinterior 304 in the direction indicated by arrow 314. Upon thesufficient build up of pressure created by the flow of the fluid, theclosed end 312 design of second member 308 forces the fluid to move inthe direction indicated by arrow 315 (best shown in FIG. 17) and throughthe flow channel defined by the threaded configuration of the twomembers 320, 308. The degree of overlap of the two threaded geometriesdetermines the hydraulic resistance, and thus the flow rate of thefluid. Therefore, the high overlap shown in FIG. 16 would result in alesser flow rate than that of the low overlap depicted in FIG. 17.Nevertheless, the fluid ultimately emerges from the resistor design asillustrated by arrows 316. It is envisioned that in other examples inaccordance with this embodiment of the present invention the shapes ofthe two members may vary, as can the particular thread design employed.

In a third embodiment the adjustment of flow rate is realized byadjusting the cross-sectional geometry of the resistor. However, unlikethe above discussed first embodiment where the cross-sectional geometryis adjusted by lengthening filament 52 in order to decrease itsdiameter, this third embodiment varies the cross-sectional geometry of atube 402 by changing its internal pressure. Once again, the thirdembodiment will be discussed with respect to pump 20; however, it may beutilized in combination with any implantable pump. As shown in FIGS.18-20, in accordance with this third embodiment, resistor 32 includes anelastic tubular element 402 disposed in a capillary 404. As best shownin FIG. 20, the tubular element 402 extends through capillary 404 and isfixed at its ends by sealing elements 406 and 408. As shown in FIGS. 18and 20, the tubular element 402 is situated so as to define aring-shaped flow channel 410 through capillary 404. However, like theabove discussed first embodiment, the tube may be positionedeccentrically, thereby forming a sickle-shaped flow channel 410, asshown in FIG. 19.

In operation, fluid flows in the direction indicated by arrows F, and issubjected to the flow channel from entrance 412 to exit 414. Once again,the effective length of the resistor extends along the portion wheretube 402 and capillary 404 overlap. The diameter of tubular element 402depends upon its internal pressure P1. Thus, the flow rate of the fluidcan be affected by pressure being applied or reduced to the inside oftube 402. Rising the pressure will increase the outer diameter of thetubing and thus will have the effect of reducing the flow rate.Similarly, lowering the pressure will decrease the outer diameter of thetubing and increase the flow rate. It is noted that tubular element 402will have a particular resting diameter (i.e.—with no pressure beingapplied). The design of this third embodiment will be subject to theflow rate calculations discussed above in relation to the firstembodiment. Specifically, in the design shown in FIG. 19, adjusting thetubing between approximately eighty five percent (85%) to ninety percent(90%) of the overall inner diameter of capillary 404 will result in anapproximate flow rate variation of 1 to 25, which is the desired ratiofor an implantable pump. However, it is to be understood that theoperation of this third embodiment will be substantially opposite tothat of the first embodiment. Clearly, rather than decreasing thediameter of tube 402 from its resting diameter, this third embodimentaims to increase the diameter. Thus, operation of tube 402 will move thesystem from a state in which the flow rate is greater to a state wherethe flow rate is lesser. This is contrary to the first embodiment.

Any means suitable for rising and lowering the pressure to the inside oftubular element 402 can be utilized. For example, it is envisioned thata piston or bellows assembly may be utilized, or that a chemicalreaction may be employed to achieve the pressure differential.

In a fourth embodiment the adjustment of flow rate is realized byproviding an insert 502 having a longitudinally varying cross section.By moving the insert 504 along the longitudinal axis of a capillary 504,the hydraulic resistance of resistor 32 is changed. Once again, thefourth embodiment will be discussed with respect to pump 20; however, itmay be utilized in combination with any implantable pump. As shown inFIGS. 21-24, in accordance with this fourth embodiment, resistor 32includes the aforementioned insert 502 positioned within a capillary504. In one example of this fourth embodiment, as is shown in FIGS. 21and 23, insert 502 is depicted as having a conical shape, and iscentrally located within capillary 504. Thus, the cross section ofinsert 502 varies across its longitudinal axis and the design forms aring-shaped flow channel 506. This insert is fixed at its ends to twomovable piston-like attachments 508, 510. However, another example isshown in FIGS. 23 and 24, in which insert 502 may be positionedeccentrically resulting in a sickle-shaped flow channel 506. In thisexample, insert 502 is fixed at its ends to two movable fixations 512,514.

In operation of both examples, fluid flows in the direction indicated byarrows F, and is subjected to the flow channel from entrance 516 to exit518 (i.e.—the aforementioned effective length). While theabove-discussed equations relating to the flow rate do not necessarilyapply to this embodiment, it is clear that the width of flow channel 506may be varied by moving insert 502 in the direction of the axis ofcapillary 504. For example, as shown in FIG. 23, movement of insert 502in the direction depicted by arrow A will cause a decrease in the widthof flow channel 506, and thus a decrease in the flow rate of the fluid.Alternatively, movement of insert 502 in the direction depicted by arrowB will cause an increase in the width of flow channel 506, and thus anincrease in the flow rate of the fluid.

It is noted that the movement of insert 502 may be achieved in differentfashions depending upon the type of design utilized. For example, asshown in FIG. 23, piston-like attachments 508, 510 are preferably movedby providing a suitable pressure thereto. However, as shown in FIG. 24,movable fixations 512, 514 may also be utilized that are moved byproviding a mechanical force thereto, from source such as a hydraulic,electrical or mechanical source or the like. Various means may beemployed for providing movement to insert 502, including those discussedherein and others that would be well known to those skilled in the art.For example, once again, magnetic forces may be employed for movinginsert 502. Finally, insert 502 may include a varying cross section thatcreates a substantially smooth longitudinal surface, as shown in thefigures, or, insert 502 may be comprised of several non-congruent crosssectional portions. The latter configuration would provide an insertthat has several different stepped sections. Thus, moving a firstsection into capillary 504 having a relatively large cross section wouldmost likely reduce the flow rate, while moving a second section oflesser cross section would increase the flow rate.

In a fifth embodiment the adjustment of flow rate is realized byadjusting the cross-sectional geometry of an insert being constructed ofan electroactive polymer (EAP). For example, such an insert may beconstructed of polyanilin, polypyrrol, or the like. This type ofmaterial is also known in the art as an artificial muscle. Essentially,the diameter of this EAP insert may be changed by applying an electricvoltage thereto. In accordance with this fifth embodiment, the voltageapplied to such an EAP insert may be between approximately zero (0) andtwo (2) volts, but may be as much as seven (7) volts. Once again, thefifth embodiment will be discussed with respect to pump 20; however, itmay be utilized in combination with any implantable pump. As shown inFIGS. 25-28, in accordance with this fifth embodiment, resistor 32includes an insert 602, which is constructed of EAP, positioned withincapillary 604. FIGS. 25 and 27 show a first example where insert 602 iscentrally located in capillary 604, while FIGS. 26 and 28 show a secondexample where insert 602 is eccentrically located in capillary 604.Further, the first example includes an insert 602 with one end fixed ata stationary attachment 608 and the other end fixed at movableattachment 610, while the second example includes an insert 602 withboth ends fixed to movable fixations 612, 614.

In operation of both examples, fluid flows in the direction indicated byarrows F, and is subjected to the flow channel from entrance 616 to exit618 (i.e.—the effective length). The width of flow channel 606 may bevaried by varying the voltage between the ends of insert 602. Suchapplication of voltage causes insert 602 to lengthen, which therebyreduces its diameter. Essentially, in accordance with this fifthembodiment, insert 602 would act as an electrode, while capillary 604may act as a counterelectrode. As has been discussed several timesabove, the decrease in the diameter of an insert similar to insert 602necessarily decreases the hydraulic resistance in capillary 604 andincreases the fluid flow rate. It is noted that the calculationsrelating to the first embodiment above may be useful in determining theproper sized insert 602 for use in examples of this fifth embodimentthat utilize an insert 602 and capillary 604 that each have circularcross sections.

The various embodiments of resistor 32, in accordance with the presentinvention, should be positioned such that fluid housed in the slowrelease chamber of an implantable pump is forced to pass through it.This configuration allows for the implantable pump to operate in itsnormal fashion, with resistor 32 controlling the fluid flow rate.However, preferred constructions would situate resistor 32 such that aninjection into a bolus port or the like would not be forced to passthrough the resistor. It is typically not required to control the flowrate of a bolus injection. Rather, such an injection is often intendedto be a quick and direct application of a medication fluid. For example,as shown in FIG. 7, resistor 32 is situated so as to capture fluidflowing from chamber 24, but not fluid directly injected into bolus port46. However, other constructions are envisioned. Furthermore, where theimplantable pump is utilized to withdraw spinal fluid, it is alsocontemplated to not force such fluid through resistor 32. In the pump ofFIG. 7, withdrawal of spinal fluid would occur through bolus port 46. Assuch, the fluid would not be required to pass through the resistor.

For each of the embodiments above, providing a controlling mechanism forselectively varying the flow rate of the medication fluid is envisioned.Many different such mechanisms are well known and widely utilized withimplantable devices for implantation into a patient's body. For example,prior art devices have shown that it is possible to utilize dedicatedhard wired controllers, infrared controllers, or the like, whichcontrollers could be used in accordance with the present invention tocontrol various elements, such as motor 282, to selectively vary theflow rate of the medication fluid. U.S. Pat. No. 6,589,205 (“the '205patent”), the disclosure of which is hereby incorporated by referenceherein, teaches the use of a wireless external control. As discussed inthe '205 patent, such a wireless control signal may be provided throughmodulation of an RF power signal that is inductively linked with thepump. The '205 cites and incorporates by reference U.S. Pat. No.5,876,425, the disclosure of which is also hereby incorporated byreference herein, to teach one such use of forward telemetry or theexchange of information and programming instructions that can be usedwith the present invention to control the pump and the variousaforementioned elements that are varied in order to affect the flowrate. However, it is noted that similar external controllers may also beutilized. Such controllers can send control signals wirelessly (such asby IR, RF or other frequencies) or can be wired to leads that are nearor on the surface of the patient's skin for sending control signals.Furthermore, a pump in accordance with the present invention may includesafeguards to prevent the inadvertent signaling or improper programmingof the pump. For example, the present invention could utilize a securepreamble code or encrypted signals that will be checked by software orhardware used for controlling the pump or even dedicated only forsecurity purposes. This preamble code would prevent the inadvertentvarying of the flow rate of the fluid from the pump, from being causedby outside unrelated remote control devices or signals and by othersimilar pump controllers. Other safety precautions may be used, such aspasswords, hardware or software keys, encryption, multiple confirmationrequests or sequences, etc. by the software or hardware used in theprogramming of the pump.

The electronics and control logic that can be used with the presentinvention for control of the motors and controllably displaceableelements used to vary the flow rate may include microprocessors,microcontrollers, integrated circuits, transducers, etc. that may belocated internally with or in the implantable pump and/or externallywith any external programmer device to transmit pump programminginformation to control the pump. For example, any external programmerdevice used to allowing programming of the pump. The electronics canalso be used to perform various tests, checks of status, and even storeinformation about the operation of the pump or other physiologicalinformation sensed by various transducers.

An external programmer device may also be avoided by incorporating thenecessary logic and electronics in or near or in the implantable pumpsuch that control can be accomplished, for example, via control buttonsor switches or the like that can be disposed on or below the surface ofthe skin. Of course, necessary precautions (such as confirmation buttonpressing routines) would need to be taken so that inadvertent changingof programming is again avoided.

A specific implantable pump 700, which incorporates the above discussedreduced size designs, as well as the above discussed infinitely variabledesigns of the present invention will now be described. Essentially,pump 700 is an implantable pump having certain novel characteristics.These characteristics allow for both the relative miniaturization andeasy construction of the pump. In addition, pump 700 incorporates one ofthe aforementioned resistor 32 designs into the specific embodiment.While pump 700 is indeed one preferred embodiment for use in accordancewith the present invention, it should be clearly understood that thepump could be modified to incorporate each of the resistor 32 designsdiscussed above in many different configurations.

As shown in FIGS. 29 and 30, pump 700 includes a housing constructed ofan upper portion 702 and a lower portion 704. The housing portions arepreferably constructed of a strong polymeric material, such aspolyetherehterketone, sold under the designation PEEK by Invibio of theUnited Kingdom. Other suitable materials may also be employed.Nevertheless, the particular material should be chosen so as to becapable of forming a two part housing that can be safely assembledwithout the use of a complicated double clinch assembly, a weldingprocess or the like. Clearly, safety is a very big concern in theconstruction of any apparatus inserted into the body especially onehousing an overdose of medication solution. Heretofore, implantable pumphousings have either been constructed of a metallic material, wherein awelding process is utilized for attaching the portions of the housingtogether, or a polymeric material, wherein a complicated clinchingassembly is utilized for attaching the portions of the housing together.For example, a metallic pumpis typically constructed by welding togethertwo metallic halves of the pump housing. Similarly, as taught incommonly owned U.S. Pat. Nos. 5,814,019 and 5,836,915, a doubleclinching assembly has been previously proposed for safely attaching thehousing halves of a polymeric pump.

In accordance with the present invention, it has been discovered thatutilizing a material such as PEEK may allow for a polymeric pump housingto be constructed without the use of any of the complicated attachmentprocedures. The elimination of such extraneous elements allows for pump700 to be smaller in size. For example, the elimination of theaforementioned double clinch safety feature allows for the overall widthof pump 700 to be reduced. Further, in certain embodiments, this mayalso decrease the overall weight of the pump, as well as the level ofcomplicity required in assembling same. As shown in FIG. 29, portions702 and 704 of the housing of pump 700 are constructed of PEEK anddesigned so as to be capable of simply screwing together. Moreparticularly, portion 702 includes an interiorly threaded extension 703for receiving an exteriorly threaded surface 705 of portion 704. Incertain embodiments, in addition to the threaded connection, a layer ofglue or other adhesive may be applied to the connection between portions702 and 704. Such an application may provide further assurance that thetwo portions do not inadvertently become detached. It is alsocontemplated that other less complicated attachment modes may beemployed. For example, in addition to the threadable connection betweenportions 702 and 704, a single clinch connection may be utilized. Inthis type of attachment, the two portions may include elements that aredesigned so as to snap fit together, and thereafter fixably secure theportions together.

As with the aforementioned generic pump 20 design, implantable pump 700further includes an interior having two chambers 724 and 726, eachchamber being separated by a flexible membrane 728. Chamber 724 isdesigned to receive and house an active substance such as a medicationfluid, while chamber 726 is designed to house a propellant that expandsisobarically under constant body temperature. Similar to above discussedgeneric pump 20, the expansion of the propellant in pump 700 displacesmembrane 728 such that the medication fluid housed in chamber 724 isdispensed into the body of the patient through the path defined by anoutlet opening 730 (FIG. 30), a cylindrical recess 764, a resistor 732(FIG. 31), a cylindrical recess 766 (FIG. 29), an outlet duct 734 andultimately an outlet catheter 736. Also in accordance with pump 20, pump700 further includes a replenishment port 738 covered by a first septum740, and an annular ring bolus port 746 covered by a second ring shapedseptum 748. The utility of each of these ports is substantiallyidentical to those of pump 20. For example, a passage 744 allows fluidinjected into replenishment port 738 to be introduced into chamber 724.In addition, like that of pump 20, it is envisioned that specificallydesigned injection needles and correspondingly situated septa may beemployed to increase safety, as discussed above.

Contrary to the aforementioned pump 20, pump 700 includes an undulatingmembrane 728 which cooperates with a similarly undulating interiorsurface 707 of portion 702. As best shown in FIGS. 29 and 30, interiorsurface 707 of portion 702 has an undulating surface that serves as thetop surface of chamber 724, while membrane 724 has a correspondingundulating surface that serves as the bottom surface of chamber 724.When chamber 724 is empty, membrane 724 fits flush against the similarlyshaped interior surface 707. This is best shown in FIG. 29. However,upon introduction of a fluid into chamber 724, membrane 728 is capableof flexing and allowing for the expansion of chamber 724. This is bestshown in FIG. 30. This undulating configuration of membrane 728 andinterior surface 707 of portion 702 allows for replenishment port 738and septum 740 to be situated at a lower position with respect to theheight of the pump. Essentially, a center portion of both interiorsurface 707 and membrane 728 are a convex shape allowing for portion 738and septum 740 to be set lower. At the same time, portions to the leftand right of this center portion are enlarged, taking substantiallyconcave shapes. This allows for the overall volume of chamber 724 toremain substantially similar in comparison to well-known implantablepumps. Operation of pump 700 also remains substantially similar to priorart implantable pumps being driven by a propellant. While the specificundulating design (i.e.—a convex or lower portion flanked by two concaveor higher portions), shown in FIGS. 29 and 30, is one suitableembodiment, other embodiments are envisioned. For example, other pumpsmay include surfaces and membranes that have corresponding shapes havingmultiple concave and/or convex portions.

The specific construction and cooperation of resistor 732 within pump700 is shown in detail in FIGS. 29-31. The resistor shown in thisspecific embodiment is akin to the above described first embodimentresistor. As best shown in FIG. 31, resistor 732 includes an elastic andresilient filament 752 situated in a capillary 754. Filament 752 extendsthrough capillary 754 and is attached on its ends to two spools 760 and762. Spool 760 resides within cylindrical recess 764 in fluidcommunication with opening 730 in portion 702, while spool 762 resideswithin a cylindrical recess 766 in portion 702. Recess 764 is in fluidcommunication with outlet opening 730 and hence chamber 724 (best shownin FIG. 30). Similarly, recess 766 is in fluid communication with outletduct 734, and hence outlet catheter 736 (best shown in FIG. 29). Thus,fluid will flow from chamber 724 through resistor 732, and out ofcatheter 736 to a target site within the body.

As best shown in FIG. 31, capillary 754 is preferably curved so as toforce filament 752 to one side thereof. Spools 760 and 762 are adaptedto wind filament 752 thereon and thus vary its cross section. As morespecifically discussed above, this varying in cross section varies theflow rate of fluid through capillary 754. In the embodiment shown inFIGS. 29-31, spool 760 is adapted to remain in a fixed position, whilespool 762 is adapted to be rotated. However, in other embodiments, bothspools may be adapted to be rotated. As best shown in the crosssectional view of FIG. 29, spool 762 is mechanically coupled to severalactuation components including being coupled via an axle 770 to a wheel772. A motor 774, like that of the above mentioned X15G, is employed toprovide rotation to wheel 772. A bearing 776 or the like may aid in therotation of axle 770, by guiding and providing smooth motion to axle770. In the embodiment shown in the figure, motor 774 receiveselectrical energy and control from an electronic unit 778, which, asdiscussed above, controlled from either internally or externally of thebody.

The aforementioned actuation components are held together and withinpump 700 through a specific cooperation that is best shown in FIG. 29.Essentially, ring septum 748 and an elastic element 780 are designed tohold the actuation components to pump 700. The actuation elements arepreferably housed so as to be a single module encompassing spool 762,axle 770, wheel 772, motor 774, bearing 776 and electronic unit 778.During assembly, this module is placed into a recess on pump 700 so thatone side abuts ring septum 748. With the module in place, septum 740 isattached to portion 702 by screwing a holder 782, which holds septum740, to portion 702 of pump 700, so as to form a threaded connection783. Holder 782 is preferably constructed of PEEK material like portions702 and 704. It is also contemplated that other modes of attachment maybe employed, such as, by adhesive or a combination of adhesive andthreads. Ring 780 of elastomeric material is preferably placed betweenholder 782 and electronic unit 778, and the cooperation thereof holdsthe aforementioned module between septum 748 and ring 780. Essentially,one side of the module is designed to cooperate with septum 748(i.e.—curved cooperation), while the other side is designed to cooperatewith ring 780 (i.e.—sloped cooperation). Thus, in the fully constructedstate, the module of actuation components is essentially frictonallyattached to pump 700.

The specific embodiment shown in FIGS. 29-31 also allows for an easyconversion from a variable flow rate pump to a fixed flow rate pump. Inoperation, the manufacturer or user of the pump would simply remove theaforementioned module of actuation components. A spacer, insert or thelike may inserted into any cavity formed in the housing of pump 700,after the removal of the module. Filament 752 is also removed fromcapillary 754 and replaced with a small tube (not shown), constructed ofa material such as glass. The tube preferably has an outer diameterslightly smaller than the inside diameter of capillary 754, so as toallow a snug fit therein. Further, the tube may have any suitable innerdiameter, it being noted that the particular inner diameter sizedictates the flow rate of fluid through capillary 754. Thus, dependingupon the desired fixed flow rate, a particular tube having a suitableinner diameter should be selected. Finally, the tube should be capableof conforming to the preferable curved shape of capillary 754. Withthese simple modifications to pump 700, a relatively inexpensive fixedflow rate pump may be produced. This simple conversion allows for theuse of the majority of the components of pump 700 without requiring themodification of any. This is beneficial, because new molds or the likewould not be needed to change between pump designs.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. An implantable device for dispensing an active substance to a patientcomprising: a housing defining a chamber, said housing having an outletfor delivering said active substance to a target site within saidpatient, said outlet in fluid communication with the chamber; and EAPmeans for varying a flow rate of said active substance between saidchamber and said outlet.
 2. The implantable device according to claim 1,wherein said EAP means includes an insert.
 3. The implantable deviceaccording to claim 2, wherein said insert is constructed frompolyanilin.
 4. The implantable device according to claim 2, wherein saidinsert is constructed from polypyrrol.
 5. The implantable deviceaccording to claim 2, wherein said EAP means further includes a powersource.
 6. The implantable device according to claim 5, furthercomprising means for controlling said EAP means for varying the flowrate of said active substance between said chamber and said outlet. 7.The implantable device according to claim 6, wherein said means forcontrolling said EAP means are capable of selectively applying betweenzero (0) and two (2) volts.
 8. An implantable device for dispensing anactive substance to a patient comprising: a housing defining a chamber,said housing having an outlet for delivering said active substance to atarget site within said patient, said outlet in fluid communication withthe chamber; a capillary in fluid communication between said chamber andsaid outlet, said capillary having an inner surface; and a flow controlelement received within said capillary, said element having an outersurface opposing said inner surface of said capillary definingtherebetween a passageway for the flow of said active substancetherethrough, said outer surface of said element moveable relative tosaid inner surface of said capillary to alter the flow of said activesubstance therethrough, wherein said flow control element is constructedof EAP.
 9. The implantable device according to claim 8, wherein saidflow control element is an elongated filament constructed from an EAPhaving a cross sectional dimension, the filament capable of beingelongated to reduce the cross sectional dimension.
 10. The implantabledevice according to claim 9, wherein said elongated filament isconstructed from polyanilin.
 11. The implantable device according toclaim 9, wherein said elongated filament is constructed from polypyrrol.12. The implantable device according to claim 9, further comprisingmeans for elongating the filament.
 13. The implantable device accordingto claim 12, wherein said means for elongating the filament includes apower source.
 14. The implantable device according to claim 13, whereinsaid means for elongating the filament is capable of selectivelyapplying between zero (0) and two (2) volts to the filament.